PHASE-TRANSFORMATION IN CARBON-COATED NITINOL, WITH APPLICATION TO THE DESIGN OF A PROSTHESIS FOR THE RECONSTRUCTION OF THE ANTERIOR CRUCIATE LIGAMENT

Ultra-low temperature isotropic carbon was vapour-deposited on a near equiatomic Ti-Ni (Nitinol) alloy (49.9 at % Ti-50.1 at % Ni). The objective of the carbon coating was the induction of growth of collagenous tissue, as part of a preliminary design of a prosthesis for the reconstruction of the anterior cruciate ligament. Differential scanning calorimetry was used to study the phase transformation of carbon-coated Nitinol. X-ray diffraction and Auger electron spectroscopy were used in the study of the carbon/Nitinol interface. The results show that unannealed coatings do not inhibit the Nitinol phase transformation regardless of the thickness of the coatings. However, on heat treating the coated samples, a TiC layer forms at the carbon/Nitinol interface. The thickness of that layer increases with increasing the time of heat treatment. Surface constraint of the Nitinol by the TiC results in a marked drop in the austenitic start (A(s)) and martensitic finish (M(f)) temperatures of thin samples. The inhibition of both the premartensitic and martensitic transformations increases with the increase in thickness of the TiC interface.